VIENNA, Feb 20 (Reuters) - Iran's stockpile of higher-enriched uranium has declined significantly for the first time in four years after it stopped such enrichment activity under a landmark nuclear deal with world powers, the U.N. atomic agency reported on Thursday.

The report by the International Atomic Energy Agency (IAEA) also indicated that Iran was meeting its commitments under the Nov. 24 pact with the powers to curb its most proliferation-prone nuclear work in exchange for some easing of sanctions.

It was issued to member states just hours after Iran and the six big powers - the United States, France, Germany, Britain, China and Russia - wound up a first round of negotiations in Vienna aimed at a final settlement of the decade-old dispute over Tehran's nuclear programme. The next round is slated for March 17.

U.N. nuclear inspectors are playing a critical role in monitoring that Iran is living up to its side of last year's six-month accord, designed to buy time for the negotiations on a comprehensive agreement over atomic activity Tehran says is entirely peaceful but the West fears may have military aims.

Iran's reserve of uranium refined to a 20 percent fissile concentration fell to 161 kg in February from about 196 kg in November, the IAEA report said. Iran suspended higher-grade enrichment under the November deal and has since converted some of the material into oxide and diluted some to a lower purity.

However, the stock of low-enriched - or 5 percent purity - uranium rose to 7,609 kg from 7,154 kg in November. This apparently resulted from a delay in constructing a plant to convert some of the material into a form less suitable for processing into high-enriched bomb material.

Iran's holding of uranium - which Tehran says it needs to fuel a planned network of nuclear power plants - is closely watched in the West as it could provide weapons-suitable material if refined much further. Iran denies any such goal. (Reporting by Fredrik Dahl; Editing by Mark Heinrich)